In one aspect, the invention relates to flow control. In another aspect, the invention relates to flow control in standpipes. In yet another aspect, the invention relates to control of catalyst circulation in a fluid catalytic cracking unit.
Standpipes are used to provide seals between processing units having solids flow therebetween. In the petroleum refining industry, for example standpipes are frequently used to provide a seal between the regenerator and the riser in a fluid catalytic cracking unit. The risk of reaction between oxygen from the regenerator and oil vapor from the bottom of the riser is thus maintained at a low level. In the plastics industry, for example, standpipes are used to prevent commingling between hydrocarbon diluent used in the polymer particle formation process and conveying gas such as nitrogen used in pneumatic polymer conveying systems.
A problem encountered in such systems is that the slide valve typically has little influence over the pressure drop across it. The pressure upstream of the slide valve is largely fixed by the solids particle head. The pressure downstream of the slide valve is largely fixed by the educator pressure downstream of the valve. Thus, the slide valve must operate at a constant pressure drop between two fixed pressures over which the slide valve has little or no control. Usually, a certain minimum pressure drop is desirable in practice for flow stability. The maximum pressure drop is determined by particle throughput requirement and other factors such as wear and erosion on the valve.
When circumstances arise such that the pressure drop available is close to the minimum desirable, such as where the unit has been retrofitted, the unit can be in a very "tight" operating mode, unless there is a means available to increase its operating flexibility. I have discovered such a means.